1. Field of the Invention
The present invention relates to novel mixtures of low molecular weight (LMW) polysaccharides and, more especially, to novel mixtures of LMW heparinic polysaccharides well adopted for the prevention of venous thromboses.
2. Description of the Prior Art
The heparins are biologically active agents of the glycosaminoglycan family, extracted from natural sources, and have valuable anticoagulant and antithrombotic properties. In particular, they are useful in the treatment of postoperative venous thromboses. However, in their native state, the heparins present a number of disadvantages which limit the extent of their effective use. Indeed, the marked anticoagulant activity of the heparins can cause hemorrhaging, and their sensitivity to certain serum factors such as pf4 mandates the administration of relatively large doses thereof. Hence, it is necessary to favor the antithrombotic activity, attributed, notably, to the antiprothrombinase activity, at the expense of the anticoagulant activity, attributed to the antithrombin effect.
Thus, it is known to this art to fragment the heparins into molecules of lower average molecular weights. For example, European Patent EP 40,144 describes the preparation of mixtures of sulfated polysaccharides of which heparin is comprised, including an ethylenic double bond at one end of their polymer chains and having a weight average molecular weight ranging from 2,000 to 10,000 daltons. These mixtures are produced by depolymerization and saponification of a heparin ester. They are said to have high antithrombotic activity and an overall anticoagulant activity lower than that of heparin.
However, one of the principal difficulties associated with the heparins is that they are very heterogeneous products. Therefore, it is difficult to assess the contribution of each of the species to the activity of heparin, to determine the behavior of these species during depolymerization and, finally, to control the structure of such species and their respective proportions in the final products. For these reasons, it has not to date been possible to resolve completely satisfactorily the difficulties indicated above. In particular, the processes described in the prior art, and especially in EP 40,144, do not permit the production of mixtures possessing the requisite pharmacological properties for improved therapeutic applications, namely, a sufficiently long plasma half-life, a fairly high absorption rate, a high bioavailability or, alternatively, a low clearance.
Other processes are also known to this art for the fragmentation of heparin with a view towards diminishing the adverse effects thereof (compare Johnson et al, Thrombos.Haemostas.Stuttg., 35, 586 (1976); Lane et al, Thrombosis Research, 16, 651; Lasker et al, U.S. Pat. No. 3,766,167)). Each appears to indicate that the desired activity is favored when the degree of fragmentation of heparin increases (see also published European Patent Application EP 301,618 relating to pentasaccharides possessing antithrombotic activity).
Likewise, recent studies on the mechanism of action of the heparins in thrombin formation have demonstrated an influence of the average molecular weight of the heparins on their activity in vitro (Beguin et al, Thromb. Haemost., 61, 30 (1989)). It is reported that low molecular weight heparins tend to possess an antiprothrombinase activity, and heparins of higher molecular weight an antithrombin activity.
It too has been proposed to this art to fractionate the heparins in order to extract mixtures of more homogeneous average molecular weight therefrom. European Patent Application EP 337,327 thus describes a process for preparing oligosaccharide fragments derived from heparin, permitting mixtures having a reduced molecular weight dispersion to be obtained. According to this process, the fractions having a molecular weight below 3,000 daltons are first removed, whereby the final product is devoid of fragments containing less than 10 to 16 saccharides, and then the species having a molecular weight above 7,000 daltons. This treatment is said to provide a more homogeneous final mixture having decreased anticoagulant activity while at the same time preserving the desired antithrombotic activity.
Nonetheless, these final mixtures continue to elicit a residual hemorrhagic effect, or too low an antithrombotic response. In addition, the prior art is essentially silent in respect of which particular properties have to be combined to provide optimum biological activity. This is reflected in the above publication, where the authors conclude, "We do not know what combination of the properties of heparin is optimal. The precise characterization of different preparations and the correlation of these properties with clinical observations might possibly provide an answer."